Will be in Canada for all of September - flying in and out of Toronto. I'm thinking initially getting flixbus from airport and go down to Niagra for a few days then get bus back and then pickup a car.
I only want to stay in Canada - USA is off the table. A shame as original plan we probably would have went to arcadia and Bay of Fundy etc. We were in Nova Scotia last year so our sole exposure to Canada. Suspect we will organise another one at somepoint that will do Vancouver, Banff etc.
Any recommendation for road trip - we don't want to do anything crazy and be driving for hours every day. More drive a few hours - spend a few days then repeat.
Would we need to book accomadation in advance or is it ok to wing it

Sounds like a great plan, and September is one of the best months to be in Ontario — fewer crowds, great weather, and early fall colours starting to show.

Your Niagara idea makes sense. A couple of days there is plenty (Niagara Falls + Niagara-on-the-Lake + a winery stop). Once you’re back in Toronto and pick up a car, you’ve got some really relaxed road-trip options that don’t involve endless driving.

Easy Ontario road-trip loops (no crazy drive days)

Toronto → Prince Edward County (2–3 nights)

About 2–2.5 hrs drive. Great food scene, wineries, beaches (Sandbanks), and a very chilled pace.

Prince Edward County → Kingston (1–2 nights)

Short hop (1 hr). Lovely historic town, waterfront walks, Fort Henry, good cafés.

Kingston → Algonquin area / Huntsville (2–3 nights)

Around 3 hrs. Classic Canadian lakes, canoeing, wildlife, and great hiking. September is perfect here.

Algonquin → Muskoka (1–2 nights)

Only 1–1.5 hrs. More lakes, scenic drives, and relaxed cottage country vibes.

Muskoka → Tobermory & Bruce Peninsula (2 nights)

About 3 hrs. Stunning turquoise water, hiking in Bruce Peninsula NP, Georgian Bay coastline.

Then you can loop back to Toronto in ~4 hrs without feeling rushed.

The plan from Mrshinewash is a good one, seeing quite a bit of Ontario. Another option or extension would be to go up to Ottawa after Kingston; our nations capital with plenty of museums From there loop into Quebec heading towards Montreal and Quebec city, looping back through Quebec's Eastern Townships area. In later September you may start to see fall colours through this area.